Re: [PATCH 11/11] mm/page_alloc: Embed per_cpu_pages locking within the per-cpu structure

[Mel Gorman <mgorman@techsingularity.net>]

On Thu, Apr 15, 2021 at 04:53:46PM +0200, Vlastimil Babka wrote:
> On 4/14/21 3:39 PM, Mel Gorman wrote:
> > struct per_cpu_pages is protected by the pagesets lock but it can be
> > embedded within struct per_cpu_pages at a minor cost. This is possible
> > because per-cpu lookups are based on offsets. Paraphrasing an explanation
> > from Peter Ziljstra
> > 
> >   The whole thing relies on:
> > 
> >     &per_cpu_ptr(msblk->stream, cpu)->lock == per_cpu_ptr(&msblk->stream->lock, cpu)
> > 
> >   Which is true because the lhs:
> > 
> >     (local_lock_t *)((zone->per_cpu_pages + per_cpu_offset(cpu)) + offsetof(struct per_cpu_pages, lock))
> > 
> >   and the rhs:
> > 
> >     (local_lock_t *)((zone->per_cpu_pages + offsetof(struct per_cpu_pages, lock)) + per_cpu_offset(cpu))
> > 
> >   are identical, because addition is associative.
> > 
> > More details are included in mmzone.h. This embedding is not completely
> > free for three reasons.
> > 
> > 1. As local_lock does not return a per-cpu structure, the PCP has to
> >    be looked up twice -- first to acquire the lock and again to get the
> >    PCP pointer.
> > 
> > 2. For PREEMPT_RT and CONFIG_DEBUG_LOCK_ALLOC, local_lock is potentially
> >    a spinlock or has lock-specific tracking. In both cases, it becomes
> >    necessary to release/acquire different locks when freeing a list of
> >    pages in free_unref_page_list.
> 
> Looks like this pattern could benefit from a local_lock API helper that would do
> the right thing? It probably couldn't optimize much the CONFIG_PREEMPT_RT case
> which would need to be unlock/lock in any case, but CONFIG_DEBUG_LOCK_ALLOC
> could perhaps just keep the IRQ's disabled and just note the change of what's
> acquired?
> 

A helper could potentially be used but right now, there is only one
call-site that needs this type of care so it may be overkill. A helper
was proposed that can lookup and lock a per-cpu structure which is
generally useful but does not suit the case where different locks need
to be acquired.

> > 3. For most kernel configurations, local_lock_t is empty and no storage is
> >    required. By embedding the lock, the memory consumption on PREEMPT_RT
> >    and CONFIG_DEBUG_LOCK_ALLOC is higher.
> 
> But I wonder, is there really a benefit to this increased complexity? Before the
> patch we had "pagesets" - a local_lock that protects all zones' pcplists. Now
> each zone's pcplists have own local_lock. On !PREEMPT_RT we will never take the
> locks of multiple zones from the same CPU in parallel, because we use
> local_lock_irqsave(). Can that parallelism happen on PREEMPT_RT, because that
> could perhaps justify the change?
> 

I don't think PREEMPT_RT gets additional parallelism because it's still
a per-cpu structure that is being protected. The difference is whether
we are protecting the CPU-N index for all per_cpu_pages or just one.
The patch exists because it was asked why the lock was not embedded within
the structure it's protecting. I initially thought that was unsafe and
I was wrong as explained in the changelog. But now that I find it *can*
be done but it's a bit ugly so I put it at the end of the series so it
can be dropped if necessary.

-- 
Mel Gorman
SUSE Labs